Don't touch Python state from destructors that outlive the interpreter

[alexander-beedie]

Fixes #531.

(I have a very similar issue: getting "gilstate_tss_set" crashes after running xdist pytests that use DuckDB and register custom UDFs, so was actively looking into this).

Problem

DuckDBPyModuleState is a function-local static, so its members can be destroyed after the interpreter is unusable; two destructors touch Python state from there:

• DefaultConnectionHolder::~DefaultConnectionHolder drops a shared_ptr<DuckDBPyConnection>, whose destructor enters nb::gil_scoped_release → PyEval_SaveThread.

• PythonImportCache::~PythonImportCache acquires the GIL to clear its refs.

Both abort or segfault at shutdown (depending how the process ends).

Fix

Neither destructor should touch the interpreter once it is unreachable. So, the default connection is now leaked deliberately, and the import cache releases its references without freeing them. This cleanly resolves the crashes.

Tests

Added three new test cases in tests/fast/test_module.py that can exercise each of the previously-crashing paths; each gets run by a subprocess.

• Deleting _clean_default_connection forces teardown from static destruction after Py_Finalize.
• Exiting via exit(0) / ExitProcess(0) with no GIL held and no Py_Finalize.
• A normal exit still clears the import cache, so the nanobind leak check stays quiet.